Polymeric films are used in a wide variety of applications. One particular use of polymeric films is in mirrors and polarizers that reflect light of a given polarization and wavelength range. Such reflective films are used, for example, in conjunction with backlights in liquid crystal displays to enhance brightness and reduce glare of the display. A polarizing film may be placed between the user and the backlight to direct the light towards the user and to polarize the light; thereby reducing the glare. In addition, a mirror film may be placed behind the backlight to reflect light towards the user; thereby enhancing brightness. Another use of polarizing films is in articles, such as sunglasses, to reduce light intensity and glare.
One type of polymer that is useful in creating polarizer or mirror films is a polyester. One example of a polyester-based polarizer includes a stack of polyester layers of differing composition. One configuration of this stack of layers includes a first set of birefringent layers and a second set of layers with an isotropic index of refraction. The second set of layers alternates with the birefringent layers to form a series of interfaces for reflecting light. The polarizer may also include one or more non-optical layers which, for example, cover at least one surface of the stack of layers to prevent damage to the stack during or after processing. There are other configurations that may also be used in polarizer/mirror films including stacks of layers with two or more different sets of birefringent and/or isotropic layers.
The properties of a given polyester are typically determined by the monomer materials utilized in the preparation of the polyester. A polyester is often prepared by reactions of one or more different carboxylate monomers (e.g., compounds with two or more carboxylic acid or ester functional groups) with one or more different glycol monomers (e.g., compounds with two or more hydroxy functional groups). Each set of polyester layers in the stack typically has a different combination of monomers to generate the desired properties for each type of layer. There is a need for the development of polyester films for use in polarizers and mirrors which have improved properties including physical properties, optical properties, and lower manufacturing cost.